A turn toward radical Islam by Tamerlan Tsarnaev, the Boston Marathon bombing suspect killed by police, forced a split in the family and shattered his relationship with his parents. Jennifer Smith and Alan Cullison explain. Photo: AP Images.

Speaking on talk shows, Democrats and Republicans raised doubts about the Federal Bureau of Investigation's handling of the case, especially given that Mr. Tsarnaev traveled to Russia in 2012 for six months. Several speculated that he could have been trained by extremist groups.

"If he was on their radar and they let him go…why wasn't a flag put on him?" Rep.
Michael McCaul
(R., Texas), the chairman of the House Homeland Security Committee said on CNN's "State of the Union."

The FBI said it interviewed Mr. Tsarnaev in 2011 at the request of a foreign government—which officials identified as Russia—but the FBI said it didn't find evidence of suspicious activity and closed the case.

Photos: Camaraderie After Boston Chaos

People gathered Sunday on Boylston Street at a memorial for victims of the Boston Marathon bombings. Nicolaus Czarnecki/Zuma Press

Map: Boston Area

Timeline: Terror in the U.S.

Rep.
Mike Rogers
(R., Mich.) chairman of the House Permanent Select Committee on Intelligence, said the period of Tamerlan Tsarnaev's return to Russia was when "he may have received training on what we ultimately saw last Monday" at the Boston Marathon.

That trip to Russia, Mr. Rogers said on NBC's "Meet the Press," "becomes incredibly important" and may have provided "that final radicalization to push him to commit acts of violence."

Speaking on CNN, Sen.
Charles Schumer
(D., N.Y) generally praised the FBI's handling of terrorist probes but questioned why Mr. Tsarnaev wasn't interviewed by officials when he returned to the U.S. "There were things on his website that indicated that he had been radicalized," Mr. Schumer said. "I think there's a lot of questions that have to be answered."

"The ball was dropped," said Sen.
Lindsey Graham
(R., S.C.), also on CNN. "The FBI missed a lot of things…There was a lot to be learned from this guy."

Mr. Graham added, "We're at war with radical Islamists and we need to up our game."

However, Mr. Rogers said of the FBI, "I don't think they missed anything."

He said the FBI received information from a "foreign intelligence service" which was concerned about the older brother's "possible radicalization." Officials have said the information came from Russia.

The FBI "did their due diligence and did a very thorough job about trying to run that to ground," Mr. Rogers said. But at some point, he said, "that intelligence service stopped cooperating."

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